Several approaches for disposing of waste are available today, of which the major ones are landfilling and incineration. In recent years, another technique based on supercritical water oxidation (SCWO) has been commercialized, see, e.g. Supercritical  Water Oxidation Aims for Wastewater Cleanup, C. M. Caruana, Chem. Eng. Prog., April 1995.
Supercritical water oxidation is a novel and advanced process for, inter alia, effective destruction of toxic substances, particularly organic pollutions, in wastewater and sludge. The process converts, fast and effectively, organic materials containing substantially carbon and hydrogen to carbon dioxide and water, at a temperature above the critical point of water (374° C. and 22,13 MPa), while releasing energy. The process may be completely contained and the destruction efficiency is often higher than 99%.
Heavy metals present during the process are converted to their oxides whereas sulfur and phosphorous are converted to sulfate and phosphate, respectively. Halogens are converted to their corresponding acids, e.g., hydrochloric acid. Smaller amounts of nitrogen compounds, e.g. amines and ammonia, which exist in the waste material flow, are converted to molecular nitrogen, and not to NOx, which is an acidifying and fertilizing residual product and therefore undesirable in the effluent.
If, however, the waste material contains large amounts of ammonia and/or organic nitrogen compounds, substantial amounts of the nitrogen source may be found in the effluent as ammonia as a result of the destruction process. This phenomenon is undesirable as ammonia constitutes a fertilizing compound. Besides, discharge of ammonia without further purifying is very often imposed with restrictions.
It is known in the literature, e.g. through Reactions of Nitrate Salts with Ammonia in Supercritical Water, P. C. Dell'Orco et al., Ind. Eng. Chem., Vol. 36, No. 7, 1997, and references therein, that ammonia can be converted to molecular nitrogen during supercritical water oxidation conditions if nitric acid is used as a co-oxidant together with molecular oxygen, hydrogen peroxide or another suitable compound. The nitric acid has preferably to be supplied to the waste material flow firstly after that the organic contents have been destructed with oxygen as nitrate otherwise will compete with oxygen in the destruction of the organic contents. Furthermore, the nitric acid has to be dosed with high accuracy relative to the amount of ammonia (a stoichiometric amount is needed). If too little nitric acid is supplied, a remaining amount of ammonia will be left whereas too large amounts of nitric acid will result in an excess of nitrate in the effluent.
For purposes of strength and corrosion, nickel-based alloys, such as Hastelloy or Inconel, are employed for manufacturing of equipment for SCWO. Acids, and not at least nitric acid, are, however, in presence of oxygen strongly corrosive at high temperatures, though still subcritical ones, even if these corrosion resistant nickel alloys are used, see, e.g. The Corrosion of Nickel-base Alloy 625 in Sub- and Supercritical Aqueous Solutions of HNO3 in the Presence of Oxygen, P. Kritzer et al., J. Mater. Sci. Lett., 1999, in print, and references therein. It was found in the temperature-resolved corrosion measurements reported that the corrosion due to nitric acid was most severe at temperatures between about 270° C. and 380° C., the same temperature range in which general corrosion is caused by the mixtures HCl/O2 and H2SO4/O2, respectively. At supercritical temperatures the corrosion rates were low.
For this reason, particular solutions must be employed for the entry of nitric acid into the supercritical wastewater flow containing ammonia or ammonium salts to avoid or at least minimize the corrosion.
However, as regards corrosion, generally the most troublesome compound in the supercritical water oxidation process is the chlorine element, since it is very common in various chemical substances. If the chlorine is present as an ion at elevated temperatures, it will corrode the construction materials mentioned above. The chlorine may have been an ion originally, liberated during heat up or in the reactor.
U.S. Pat. No. 5,358,645 issued to Hong et al. disclose an apparatus and process for high temperature water oxidation, the apparatus (not in detail described) having a surface area, that may be exposed to corrosive material, composed of zirconia based ceramics. The ceramics may be employed as coatings or linings.
U.S. Pat. No. 5,461,648 issued to Nauflett et al. disclose a supercritical water oxidation reactor with a corrosion-resistant lining. The inner surface of the reactor vessel is coated with artificial ceramic or diamond. A cylindrical baffle for introducing the oxygenating agent extends axially within the interior of the vessel and has its exterior surface inside the vessel coated with said artificial ceramic or diamond.
U.S. Pat. No. 5,552,039 issued to McBrayer, Jr. et al. disclose a turbulent flow cold-wall reactor. It mentions, inter alia, that if the atmosphere in the reaction chamber is harsh and corrosive, the inside wall of the reaction chamber should preferably be made of or covered with a coating or a liner withstanding the harsh atmosphere.
None of these US patents, is, however, discussing corrosion problems in terms of temperature dependent corrosivity, or the particular corrosion caused by the corrosive compounds discussed above.